1Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology, Minato, Tokyo 108-8477, 2Department of Ocean Sciences, Tokyo University of Marine Science and Technology, Minato, Tokyo 108-8477, Japan
We conducted experimental field observations to investigate the effects of the presence of adult white-spotted charr Salvelinus leucomaenis and masu salmon Oncorhynchus masou masou on the behavioral response of both juveniles in a small tributary of a Japanese mountain stream. Juveniles (small-size charr, large-size charr and large-size masu salmon) were released into three experimental areas, respectively, and their focal points, size of distribution area obtained with movement tracing and feeding frequency were recorded, under three treatments e.g., juvenile only, juvenile with an adult charr and juvenile with an adult masu salmon for each species. In only juvenile treatments, the focal points and distribution area spread all over the experiment areas for both species. In the presence of adult fishes, the focal points of juveniles shifted to near-shore margins which were significantly shallower and lower in water velocity. Moreover, the size of distribution area per fish and feeding frequency were decreased significantly. These results suggest that juveniles can be affected ecologically by the presence of adult fish in a small tributary of a Japanese mountain stream.